Type 1 pili are helical structures composed of the major pilus subunit FimA, with a distal tip composed of FimF, FimG and—at the very tip—the FimH adhesin. FimH binds to mannosylated glycoproteins on the host bladder, thereby enabling the microbe to gain a foothold for infection. Two key concepts have previously been established for pilus biogenesis mediated by the chaperone–usher pathway. First, pilus subunits comprise an incomplete immunoglobin-like fold, which lacks the seventh β-strand that is present in canonical immunoglobulin folds. As nascent pilus subunits enter the periplasm via the Sec translocon, the FimC chaperone donates its G1 β-strand to complete the immunoglobulin fold of each subunit and thereby stabilizes each subunit, in a mechanism termed donor-strand complementation. Second, pilus subunits polymerize at the FimD usher via a donor-strand exchange mechanism, in which the G1 β-strand of the chaperone is replaced by the N-terminal extension of an incoming pilus subunit. Polymerization of the pilus fibre is driven by the folding-energy differential between the β-strand insertions of donor-strand complementation and those of donor-strand exchange.The FimD usher catalyses ordered pilus biogenesis at the bacterial outer membrane. The β-barrel domain of the usher functions as a protein secretion channel and is occluded by a plug domain in the resting (apo) state. Here you can see a cryoEM structure of the FimD usher from Escherichia coli (PDB code: 6E14)

#scivis #molecularart #usher #pili #membrane #secretion #betabarrel #cryoem

Structure rendered with @proteinimaging, post-processed with @stylar.ai_official and depicted with @corelphotopaint